This invention relates to a communication system in which provision is made for the measurement of timing differences between base stations. The present invention is of particular advantage in an asynchronous CDMA (code division multiple access) telephone system.
FIG. 1 shows schematically the configuration of a typical cellular radio telecommunications system. The system comprises a number of base-stations (BSs) 1, 2, 3 etc. Each base-station has a radio transceiver capable of transmitting radio signals to and receiving radio signals from the coverage area or cell 4, 5, 6 etc. next to the base-station. By means of these signals the base-station can communicate with a mobile station (MS) 7 in that cell, which itself includes a radio transceiver. Each base station is connected to a mobile system controller (MSC) 8, which is linked in turn to the public telephone network 9. By means of this system a user of the mobile station 7 can establish a telephone call to the public network 9 via the base station in whose cell the MS is located.
It is desirable to maintain communication between the mobile station and the radio network when a mobile station moves out of the cell of a first base station and into the cell of a second base station. This is done by a handover process in which the mobile changes from the communicating with the first base station to communicating with the second base station. There are several types of handover process. In hard handover the mobile simple switches from communicating with the first base station to communicating with the second base station. This approach is used, for example, in the GSM radio telephone system. In soft handover the mobile can adopt an intermediate mode in which, when it is located in an area where the cells of the first and second base stations overlap, it communicates with both the first and second base stations—i.e. it can transfer user traffic information such as voice or data between itself and both of those base stations at the same time. This approach is used, for example, in the IS-95 radio telephone system. Soft handover is also proposed to be used in the ARIB/ETSI W-CDMA (Wideband-CDMA) system, details of the version of which are available at the ITU (International Telecommunication Union) website.
The transmissions in a radio communication system are typically organised into structural communication blocks that carry the data that is to be transmitted. For example, in the proposed W-CDMA system a superframe of 720 ms duration is divided into 72 frames of 10 ms. Each frame is divided into 16 timeslots of 0.625 ms, and each timeslot contains a number of symbols that carry the data itself. Superframes divided in this way are transmitted one after another.
In some radio communication systems the base stations are closely synchronised in time so that the transmissions of structural blocks from the base stations (such as superframes) begin at close times or closely fixed offset times. This can improve the radio performance of the system, for example, by reducing mobiles' search time (as in IS-95) and improving interference performance (as, for example, in a TDD system). For instance, in the IS-95 system the base stations are synchronised by means of the universal time information available from GPS (global positioning system) satellite transmissions. Using this universal time information all the base stations are synchronised to the chip level (typically around 1 μs or less). The timing offset of each base station is then transmitted to the mobile station. The requirement for synchronisation in this way makes base stations more difficult to deploy and more expensive to manufacture. Also, the system becomes dependant on the external timing source so the performance of the system is at the very least reduced if the external timing source is unavailable.
In other systems the base stations are unsynchronised or are only loosely synchronised. For example, the W-CDMA system is proposed to be almost totally asynchronous: the base stations are to be synchronised only to the superframe level (720 ms). This can be achieved relatively easily using the transmission network. Although this loosely synchronised approach makes it easy to deploy the base stations and reduces their cost it places high demands on the mobile stations. To cope with the looseness of the synchronisation each mobile station in the system must have a powerful synchronization search engine. The need for such powerful search engines will increase the cost of the mobile stations, which is a crucial factor in consumer-oriented mobile telephones, and increase the mobile's power consumption, reducing the standby/talk time of the mobile stations which is also a crucial issue to consumers. In addition, the synchronisation engines will take a substantial time to measure and process synchronisation information, for example for handover purposes, which may impair the operation of the mobile stations and the network—for example by delaying handovers.